At its most basic, splicing allows a node to resize the liquidity on its side of a channel (add funds / spend funds.) It does this by spending the outputs of the old channel into a new channel and adding or subtracting additional funds. The lightning spec now gives a grace period to closing channels to accommodate splices. In the the meantime (6 blocks to confirm and reannounce the newly spliced channel) the old channel state is also maintained just as a precaution and its older capacity limits must still be honored. Once the 6 blocks are up, the spiced channel can be reannounced to the network, the old channel's limitation is removed, and it's like having a new channel with updated capacity.